1. Field of the Invention
The present invention is related to the optical fiber amplifiers used in the optical communication systems and, more specifically, to an apparatus and method for compensating for the gain-spectrum-tilt of a long-wavelength band dispersion-compensating hybrid amplifier due to an environmental temperature change, and a long-wavelength band dispersion-compensating hybrid fiber amplifier equipped with the gain-spectrum-tilt compensating apparatus.
2. Description of the Prior Art
In the optical communication using wavelength-division multiplexed (WDM) systems, a plurality of channels are transmitted simultaneously; the conventional band (C-band) of 1530 to 1565 nm, the long-wavelength band (L-band) of 1570 to 1605 nm, or both of them are normally used. In these systems, the optical signals become small through the transmission fibers and recover their launching powers by the optical amplifiers at the end of each span. However, the amplified signal qualities deteriorate due to the noises, mainly amplified spontaneous emission (ASE) noise. Therefore, it is desirable to minimize the ASE noises from the optical amplifiers for the successful signal transmission. Among various optical amplifiers the erbium-doped fiber amplifiers (EDFAs) are widely used in the WDM transmissions these days because of their good gain and noise characteristics. In principle, they can have noise figures close to the quantum limit, 3 dB, if designed well. Meanwhile, most high-speed core networks based on 2.5, 10, or 40 Gbit/s per channel require dispersion compensation to keep the transmitted channels from becoming distorted in time domain, where the dispersion-compensating fibers (DCFs) are normally used. Then, the extra losses of DCFs have to be compensated by the EDFAs, which, in turn, cause the decrease of optical signal-to-noise ratios (OSNRs) of the transmitted channels. An effective way to retain high OSNRs is to incorporate the DCF into the inter-stage of a two-stage EDFA. Though, the insertion of DCF raises the noise figure of the EDFA.
An effective method for solving such a problem is to use a dispersion-compensating hybrid fiber amplifier (DCHFA). This consists of a two-stage EDFA and an actively pumped DCF where the optical signals experience Raman amplification. Unlike the C-band DCHFA, however, the L-band DCFHA has a temperature-dependent gain-spectrum tilt, which comes from the inherent temperature dependences of the absorption and emission cross sections of erbium-doped fibers (EDFs).
FIG. 1 illustrates the variation of gain spectrum according to the temperature variation of the EDFs in a conventional two-stage L-band EDFA. The gain spectrum is flat over the signal wavelengths of the L-band at 25° C. However, the gain profiles become slanted with positive gain slopes at the lower temperatures, and vice versa at the higher temperatures.
There are a few known methods to suppress these gain tilts. First, keeping the temperature of the EDF coils constant is the easiest method. However, since this method needs thermal insulation, the complete EDFA module has to become more bulky, which is against the current tendency for compact size.
The other methods are related to the control of parameters that can affect the gain spectrum, i.e., the signal powers or pump powers. As an example, U.S. Pat. No. 6,335,821 discloses a method of measuring the temperature of an EDF using a temperature sensor and controlling the pump-driving currents according to the measured temperature. In this method, the pump powers in the EDFA to get a flat gain spectrum are set. Another example is to employ a variable optical attenuator (VOA) in a two-stage EDFA. The VOA can adjust the launching signal power into the second stage, which is usually in a deep saturation regime. Since the gain spectrum of a deeply saturated EDFA changes with the input signal power, the inter-stage VOA can adjust the overall gain-spectrum tilt.
In principle, keeping the temperature of an overall optical amplifier constant can fix the temperature-dependent gain tilts in an L-band DCHFA as well as an L-band EDFA. However, the size problem of a relatively bulky amplifier still remains. On the other hands, the methods of the parameter control cannot be directly applied to the DCHFA since it has the pumped DCF section where the Raman amplification process occurs. Understanding its impact on the overall gain spectrum of an L-band DCHFA under the temperature changes should be preceded to compensate accurately the gain-spectrum tilts.